The present invention relates to a mercapto compound, a process for preparing a mercapto compound, a process for preparing a polythiourethane base resin containing the mercapto compound, and a process for preparing plastic lenses containing the resin. The mercapto compound of the invention is useful as a cross-linking agent, a hardener for epoxy resin, a vulcanizing agent, a polymerization modifier, a raw material of plastic resin, an antioxidant, a metal complex forming agent, a biochemical agent and a lubricating oil additive.